1. Field of the Invention
The present invention relates to a solid-state image pickup device, a method for manufacturing the solid-state image pickup device, and an electronic apparatus using the solid-state image pickup device.
2. Description of the Related Art
A CCD (Charge Coupled Device) type solid-state image pickup device used in an area sensor, a digital still camera or the like has a light receiving section for generating and accumulating a signal charge according to incident light and a charge transfer section for transferring the signal charge from the light receiving section. The charge transfer section has a CCD structure. The charge transfer section includes a charge transfer path formed in a semiconductor substrate and a plurality of transfer electrodes disposed so as to be adjacent to each other above the charge transfer path. The charge transfer section transfers the signal charge read out from the light receiving section by sequentially driving the plurality of transfer electrodes.
In the CCD type solid-state image pickup device, when light is made directly incident on a part under the transfer electrodes, the light becomes a so-called smear, and produces a pseudo image in an image. In order to suppress such a phenomenon, in an IT (Interline Transfer) type CCD solid-state image pickup device, a light shielding film of a material that blocks light is generally formed on transfer electrodes.
A metallic material is generally used as the material for the light shielding film. In addition, using a part of the light shielding film as a readout electrode used to read out the signal charge accumulated in the light receiving section to the charge transfer path at the same time has been proposed. An existing solid-state image pickup device using a light shielding film as a readout electrode will be described below.
FIG. 28 is a schematic sectional configuration diagram of an existing CCD type solid-state image pickup device. The existing CCD type solid-state image pickup device includes a light receiving section PD, a charge transfer path 305 formed so as to be adjacent to the light receiving section PD, a light shielding film 322 for shielding regions excluding the light receiving section PD from light, a readout electrode 320, and a transfer electrode 333. The existing CCD type solid-state image pickup device also includes a color filter layer 325 and an on-chip lens layer 326 formed on a light incidence side.
The light receiving section PD is formed on the light incidence side of a substrate 300 made of an n-type semiconductor substrate, for example. The light receiving section PD mainly includes a dark current suppressing region 302 made of a p-type high-concentration impurity region formed in the surface of the substrate and a charge accumulating region 301 made of an n-type impurity region formed under the dark current suppressing region 302. In the light receiving section PD, a main photodiode is formed by a pn junction between the dark current suppressing region 302 and the charge accumulating region 301.
The charge transfer path 305 is formed by an n-type impurity region adjacent to one side of the light receiving section PD in the substrate 300. In addition, a well region 304 made of a p-type high-concentration impurity region is formed under the charge transfer path 305.
The transfer electrode 333 is formed on the charge transfer path 305 with a gate insulating film interposed between the transfer electrode 333 and the charge transfer path 305. In actuality, a plurality of transfer electrodes 333 are formed in a vertical direction over the charge transfer path 305. The charge transfer path 305 and the transfer electrode 333 form a vertical transfer register of a CCD structure. The vertical transfer register reads out a signal charge generated and accumulated in the light receiving section PD to the inside of the charge transfer path 305, and thereafter transfers the signal charge read out to the charge transfer path 305 in the vertical direction by sequentially driving the plurality of transfer electrodes 333.
The light receiving section PD and the vertical transfer register adjacent to the light receiving section PD form a pixel. Each pixel is separated by an element isolation region 303 made of a p-type high-concentration impurity region formed in the substrate 300.
The light shielding film 322 is formed of a metallic material made of aluminum (Al), for example. The light shielding film 322 is formed over the transfer electrode 333 formed on the substrate 300 with an interlayer insulating film 311 interposed between the light shielding film 322 and the transfer electrode 333. The light shielding film 322 has an overhang part 321 formed to prevent the occurrence of a smear in the charge transfer path 305 due to light incident from an edge of the light shielding film 322.
A plasma SiN film 323 is formed on the entire surface of the substrate 300 including the light shielding film 322. A planarizing film 324 is formed on the plasma SiN film 323. A color filter layer 325 of R (red), G (green), and B (blue) is formed for each pixel on the planarizing film 324. An on-chip lens layer 326 is formed at a position corresponding to the light receiving section PD of each pixel on the color filter layer 325.
In the existing solid-state image pickup device, the overhang part 321 formed by making an edge part of the light shielding film 322 overhang to a position above a readout gate section formed between the light receiving section PD and the charge transfer path 305 is formed so as to serve also as the readout electrode 320 for an operation of reading out a signal charge.
In the solid-state image pickup device having the above-described configuration, a signal charge generated and accumulated in the light receiving section PD is read out to the charge transfer path 305 via the readout gate section by applying a readout voltage to the light shielding film 322. Then, the read-out signal charge is transferred in the vertical direction by sequentially applying a transfer voltage to the transfer electrodes 333.
In such a structure of the existing solid-state image pickup device, the light shielding film 322 can serve also as the readout electrode. It is therefore possible to increase the area of the light receiving section PD as compared with a structure retaining the light shielding film 322 and the readout electrode 320 separately from each other, and take light into the light receiving section PD more easily.
As shown in FIG. 28, when the light shielding film 322 is used as the readout electrode, the transfer electrode 333 and the readout electrode are formed by separate processes, and positional relation between the transfer electrode 333 and the readout electrode is determined by the film thickness P of the interlayer insulating film 311 formed between the transfer electrode 333 and the readout electrode. Thus, when the film thickness P of the interlayer insulating film 311 is changed, positional relation between the readout electrode 320 and a potential barrier formed between the light receiving section PD and the charge transfer path 305 within the substrate 300 is shifted.
FIGS. 29A and 29B show a potential configuration within the substrate 300 along a line A-A′ in FIG. 28.
FIG. 29A is a diagram of potential changes in a case where positional relation between the potential barrier formed between the light receiving section PD and the charge transfer path 305 and the readout electrode 320 is ideally formed. FIG. 29B is a diagram of potential changes in a case where the positional relation between the potential barrier formed between the light receiving section PD and the charge transfer path 305 and the readout electrode 320 is formed in a shifted manner.
As shown in FIG. 29A, in the case where the positional relation between the potential barrier and the readout electrode 320 is formed ideally, when a readout voltage is applied to the readout electrode 320, the voltage of the readout electrode 320 is applied to the whole of the potential barrier. Thus, the barrier can be crushed even with a low voltage, and a signal charge can be read out from the light receiving section PD to the charge transfer path 305 efficiently.
On the other hand, as shown in FIG. 29B, in the case where the positional relation between the potential barrier and the readout electrode 320 is formed in a shifted manner, when a readout voltage is applied to the readout electrode 320, the voltage of the readout electrode 320 is not applied to the whole of the potential barrier. In this case, the application of the same readout voltage as in the case where the potential barrier and the readout electrode 320 are formed ideally as in FIG. 29A cannot crush the barrier, so that the signal charge cannot be read out from the light receiving section PD to the charge transfer path 305. The example shown in FIG. 29B thus needs a high voltage to crush the barrier.
When the readout voltage is set high in consideration of variation in the positional relation between the readout electrode 320 and the potential barrier, the signal charge can be read out from the light receiving section PD to the charge transfer path 305. However, a high readout voltage means a high power consumption. It is thus desirable that the readout voltage be as low as possible.
In general, for the readout electrode 320 formed by the light shielding film 322 in the existing solid-state image pickup device described above, a film of a material having a light shielding property is formed at the time of the formation of the readout electrode 320 and thereafter patterned by a lithography method, and an excess film is removed by a dry etching method. However, in this case, there are level differences at the time of the patterning, thereby resulting in variations in the film thickness of a photoresist used in the lithography method and thus resulting in variations in pattern dimensions. Therefore the width of the readout electrode 320 also varies.
Because variations in the positional relation between the transfer electrode 333 and the readout electrode 320 and variations at the time of the patterning thus interact with each other, it is extremely difficult to control the readout voltage in the existing structure.
[Patent Document 1]    Japanese Patent Laid-Open No. 2004-165462